New revelations about BP's operations on the Deepwater Horizon oil rig on the day of the explosion are being described as the smoking gun that proves the oil giant's culpability in the disaster.

BP hired a reputable oilfield service company to test the strength of cement linings on the well, but then sent the company's workers home 11 hours before the explosion on April 20 -- "without performing a final check that a top cementing company executive called 'the only test that can really determine the actual effectiveness' of the well's seal," reports the New Orleans Times-Picayune:

A spokesman for the testing firm, Schlumberger, said BP had a Schlumberger team and equipment for sending acoustic testing lines down the well "on standby" from April 18 to April 20. But BP never asked the Schlumberger crew to perform the acoustic test and sent its members back to Louisiana on a regularly scheduled helicopter flight at 11 a.m., Schlumberger spokesman Stephen T. Harris said.

At a few minutes before 10 p.m., a belch of natural gas shot out of the well, up a riser pipe to the rig above, igniting massive explosions, killing 11 crewmembers and sending millions of gallons of crude oil into the Gulf. The rig's owner, Transocean, blames failed cement seals, installed by Halliburton, for the disastrous blowout.

BP has also come under fire for its safety procedures -- crewmember Mike WIlliams told "60 Minutes" that despite damage to the critical blowout preventer, BP ordered the rig operator, Transocean Ltd., to ignore a critical safety measure during the sealing of the well. By failing to use drilling mud, a heavy liquid that keeps oil and gas from coming back up the pipe, to seal the well, BP saved money but may have caused the explosion.

CBS News released an extended interview with Williams today, in which he describes in detail the explosion, subsequent fire and his escape from the rig.

Drilling experts say that blame probably lies with flaws in the cementing process -- plugging holes in the pipeline seal by pumping cement into it from the rig -- which was performed by Halliburton on the Deepwater Horizon.

As HuffPost reported last month: "Last year, Halliburton was also implicated for its cementing work prior to a massive blowout off the coast of Australia, where a rig caught on fire and spewed hundreds of thousands of gallons into the sea for ten weeks.

In that incident, workers apparently failed to properly pump cement into the well, according to Elmer Danenberger, former head of regulatory affairs for the U.S. Minerals Management Service, who testified to an Australian commission probing that accident."